Photonic crystal fibers (PCF), or holey fibers as they are sometime referred to, are a relatively new class of optical fibers. Most commonly, these fibers have a core surrounded by a periodic array of elongate holes formed in a matrix material. The array acts to confine light to the core. Since the core, in effect, breaks the periodicity of the array of holes, it is often referred to as a core defect.
In one class of PCF, the core defect is solid and the holes are formed in a matrix material, which is typically made from the same material as the core. The combination of the holes and the matrix material in this kind of fiber acts to create an effective refractive index in the cladding region, which is lower than the refractive index of the core defect, and the fiber can guide light in the core defect by virtue of the effective refractive index step between the core and cladding; by analogy to standard optical fiber guidance. One category of this kind of fiber is described in “Endlessly single-mode photonic crystal fiber”, Birks et al., Optics Letters 22 (961–963), 1997.
In another class of PCF, the core defect comprises a void and the holes in the cladding are arranged in a periodic array to create a photonic band-gap structure. The photonic band-gap prevents light over a range of predetermined wavelengths and propagation constants from passing through the structure. By this means, light having one of the predetermined wavelengths and propagation constants, which is coupled into the core defect, can be confined to and guided along the fiber in the core defect. One category of this kind of fiber is described in International Patent Application PCT/GB00/01249.
A number of different ways to manufacture PCF are known. Most commonly, a preform stack is assembled using individual elongate glass elements to match, in cross section and on a macro scale, the form of the desired PCF. The stack is then typically over-clad using a large diameter glass tube or capillary, which is large enough to cover the stack and small enough to keep the individual elements in their relative positions in the stack. The stack is then heated and drawn into a fiber, in one or more drawing stages, in a standard fiber drawing facility. One or more over-cladding layers of material may be added for strength and protection during the drawing stages.
A preform for a PCF having a solid core defect is typically relatively simple to assemble, since the cladding region can comprise a bundle of hollow capillaries and the core defect can comprise a solid rod, which has the same external dimensions as the hollow capillaries. The solid rod simply replaces a capillary in an inner region of the bundle.
In contrast, it can be more difficult to assemble a preform for a hollow core PCF, since the hollow core defect in a resulting fiber typically needs to have a cross sectional size that is significantly larger than any one cladding hole, in order for the core defect to be large enough to support at least one core-guided mode.
In the aforementioned patent application PCT/GB00/01249, a pre-form suitable for making a hollow core PCF was formed by omitting from a stack of round cross section silica capillaries, arranged in a close-packet triangular array, an inner capillary and, in addition, the six capillaries surrounding the inner capillary. The resulting fiber may, therefore, be referred to as having a seven-cell core defect. The capillaries around the core defect boundary in the stack were supported during formation of the pre-form by inserting truncated capillaries, which did not meet in the middle of the stack, at both ends of the capillary stack. The stack was then heated in order to fuse the capillaries together into a pre-form suitable for drawing into an optical fiber.
An alternative method for making a PCF is described in U.S. Pat. No. 6,444,133. This patent describes a preform stack comprising hexagonal capillaries having circular bores, from which an inner capillary is omitted to leave a hexagonal void. U.S. Pat. No. 6,444,133 proposes that, by etching the entire pre-form, the flat surfaces of the hexagonal void dissolve away more quickly than the curved surfaces of the exposed capillary bores. The effect of etching is that the walls of the capillaries next to the hexagonal void fully dissolve, while the remaining capillaries simply experience an increase in hole-diameter. Overall, the resulting pre-form has an increased fraction of air in the cladding structure and a core defect that is closer in size to a seven-cell core defect than to a single cell core defect.
United States Patent Application No. 2004/0050110 describes different methods for making PCF preforms. In one embodiment, a method involves providing so-called sacrificial rods, corresponding to the desired locations of holes in a desired preform, depositing soot around the rods, consolidating the soot to form a solid, structured body, removing the rods, thereby leaving holes in the preform and, finally, drawing the preform into a PCF. The present inventors believe that it would take a significant amount of time to deposit an amount of soot necessary to form a preform in the manner described.
In applicant's co-pending, published patent application WO03/080524, a method of making a PCF is described which involves drawing PCF from a preform while pressurizing different holes in the preform at different pressures. The method is suitable for drawing fibers in which the hole sizes in the preform are different or where it is desired to vary the hole size relationships in a PCF compared with the same hole size relationships in the respective preform.
In general, it has been found desirable to pressurize holes in the preform during the fiber drawing process in order to counteract the effects of surface tension in the glass (or other material) used to make the PCF, which tends to collapse the holes. It follows that it is desirable to maintain a lower pressure in larger holes in a preform than in the smaller holes, since surface tension causes smaller holes to collapse more easily than larger holes. Hence, in making hollow core PCF in particular, it is desirable to maintain during fiber drawing a pressure differential between the smaller cladding holes and a larger core hole; with the core hole being under the lower relative pressure.
One way to facilitate differential pressurization is to assemble the stack and heat and fuse the elements of the stack together before the fiber drawing step. Then, the core region is isolated from the cladding region and the stack can be differentially pressurized during the drawing step. It has, however, been found difficult to heat the stack to a temperature sufficient to fuse the capillaries without the capillaries deforming or even collapsing.
Another way to facilitate such differential pressurization is to use a large diameter capillary or tube in the core region of a preform. This method has the advantage that the large diameter capillary acts both as a natural support member, to support the cladding capillaries around the core defect region, and as the means for isolating the core region from the cladding holes. While this method finds useful application in PCF manufacture, it naturally adds material to the periphery of the core defect region, which would otherwise comprise material only from the cladding capillaries. The present inventors have found that a thicker core boundary can lead to deleterious effects, such as becoming a means for supporting unwanted surface modes, which are guided on or near to the surfaces of the core boundary. These surface modes can become loss paths, due to undesirable coupling of power from core-guided modes to the surface modes.
An additional problem with using a large diameter capillary in a core region is that it can be difficult to match the outer cross sectional shape of the capillary, at the dimensions required in a stack, with the cross sectional shape of the void left by omitting one or more capillaries. For example, if seven round capillaries are removed, as in WO03/080524 referred to above, the core void 10 has a generally hexagonal cross sectional shape, as shown in the schematic diagram in FIG. 1. A large diameter capillary 12 can be formed to match the cross section of the void, as shown in FIG. 2. Alternatively, a round, large diameter capillary 14 may be used, as shown in FIG. 3.
In the former case, accurately forming a capillary 12 with a cross section that is not round can be difficult, particularly if the capillary needs to be drawn down to a different scale after initial formation. In the latter case, unwanted interstitial voids 16 form between the large diameter capillary and the cladding capillaries. While the unwanted voids can be removed by evacuating them during a fiber drawing step, such evacuation can result in undesirable core boundary deformation, caused by portions of the large diameter capillary and portions of the cladding capillaries, around the unwanted interstitial voids, deforming towards one another as the voids close. In this event, a resulting PCF structure may acquire different sizes and shapes of hole around the core defect, which can disrupt the band-gap, for example by reducing its bandwidth of operation.